Unify the minimum TLS version of DoH and DoT

[PeterDaveHello]

Currently, TLS 1.0 and TLS 1.1 is not offered by blocky's DoT implementation, but offered by blocky's DoH implementation.

As DoH and DoT are both still something new, and there're security concerns about TLS < 1.2, maybe should unify both of their minimum TLS version to v1.2 here?

Using testssl.sh(https://github.com/drwetter/testssl.sh) to test the TLS version.

On https port:

 Testing protocols via sockets except NPN+ALPN 

 SSLv2      not offered (OK)
 SSLv3      not offered (OK)
 TLS 1      offered (deprecated)
 TLS 1.1    offered (deprecated)
 TLS 1.2    offered (OK)
 TLS 1.3    offered (OK): final
 NPN/SPDY   not offered
 ALPN/HTTP2 h2, http/1.1 (offered)

On TLS port:

 Testing protocols via sockets except NPN+ALPN 

 SSLv2      not offered (OK)
 SSLv3      not offered (OK)
 TLS 1      not offered
 TLS 1.1    not offered
 TLS 1.2    offered (OK)
 TLS 1.3    offered (OK): final
 NPN/SPDY   not offered
 ALPN/HTTP2 not offered

[PeterDaveHello]

PR #524 sent (originally waiting for #520 to be merged first)